Millions of Americans will spend the weekend hot under the collar as soaring East Coast temperatures and stifling humidity slapped excessive weather warnings on New York and Philadelphia.

The combination of heat and humidity would make it feel as hot as 110 degrees Fahrenheit (43 degrees Celsius) in the two cities.

Temperatures would stay in the mid 90s from Friday through Sunday, with the humidity pushing heat index values higher in New York and Philadelphia, home to around 10 million people combined, meteorologists said.

Authorities warned of heat-related health problems, especially for the elderly and those with chronic health problems, and for people who work outdoors.

Americans were advised to stay inside and use air conditioning where possible, check on vulnerable friends and neighbors, drink plenty of fluids to stay hydrated, and not leave children or pets unattended in vehicles.

Overall, the heatwave stretched from southwest Ohio to western Virginia and Washington, and north through Philadelphia, New York and Boston, said David Roth, a meteorologist with the National Weather Service.

Dallas, Washington, parts of New Jersey and New York's Long Island could hit record highs on Friday, Roth told AFP.

Five other spots stretching from Maryland's Ocean City to Connecticut might set similar records on Saturday, before the heat fades Sunday and Monday, he added.

In the southern US, heavy rain pummelled parts of Louisiana and Mississippi as an area of low pressure slowly moved west along the Gulf Coast. Many streams were out of their banks on Friday, the National Weather Service said.

The area recorded 10 to 15 inches (25.4 to 38.1 cm) of rain, Roth said. Another 10 inches were expected in parts of Louisiana over the next two days.

"Even for them it's very unusual," he told AFP.

Louisiana Governor John Bel Edwards declared a state of emergency and planned to hold a press conference Saturday morning after meeting with emergency officials.

"We are in constant contact with local officials and first responders, and assistance is already on the move to affected parishes," Edwards said in a press release.

One man in the capital city of Baton Rouge was killed in the high waters, and many roads were closed due to flooding, local television station WAFB reported. Schools were closed.

The National Weather Service warned of "continued significant flash flooding potential."

Entergy Louisiana said more than 7,500 customers were without power Friday night.